Means to directionally launch the te01 mode in a circular waveguide from the te10 mode in a rectangular waveguide



June 13, 1967 R. M. PHILLIPS 3,325,751 MEANS TO DIRECTIONALLY LAUNCH THE TE MODE IN A CIRCULAR WAVEGUIDE FROM THE TE MODE IN A RECTANGULAR WAVEGUIDE Filed Aug. 4, 3.964

INVENTOR ,e. 2 Paul ms BY 17770.8 y

lice-4 7- United States Patent MEANS TO DIRECTIONALLY LAUNCH THE TE MODE IN A CIRCULAR WAVEGUIDE FROM THE TE MODE IN A RECTANGULAR WAVE- GUIDE Robert M. Phillips, Redwood City, Calif., assignor to the United States of America as represented by the Secretary of the Air Force Filed Aug. 4, 1964, Ser. No. 387,536 2 Claims. (Cl. 333-21) The purpose of this invention is to provide means for directionally launching the TE mode in a circular waveguide from the TE mode in a rectangular waveguide that meets the following requirements: (1) that it have a broad bandwidth, (2) that the mode be launched in a minimum distance along the circular waveguide, and (3) that the center of the circular waveguide be left unobstructed for the passage of an electron beam.

Briefly, the above is accomplished by providing four similar sections of rectangular waveguide equally spaced about the circular waveguide and extending through its wall at an angle of 45 to its axis. The larger dimension of the rectangular sections is chosen to provide the same cut-off frequency as that of the circular guide. The smaller dimension of the rectangular waveguide sections is normal to the circular waveguide axis and is so chosen that the four waveguide sections in parallel have the same characteristic impedance as the circular waveguide operating in the TE mode. In order to allow for passage of a beam along the axis of the circular waveguide, the pyramid that would normally be formed by the intersection of the four rectangular sections inside the circular waveguide is removed. A four-way power splitter is employed to divide the power in the main rectangular waveguide into four equal components which are applied to the rectangular sections in proper phase to excite the TE mode in the circular waveguide.

The invention will be described in more detail with reference to the accompanying drawings in which:

FIG. 1 shows a section of the mode launcher taken along the section line 11 of FIG. 2;

FIG. 2 is a sectional view of the mode launcher looking in the direction 22 of FIG. -1;

FIG. 3 shows the method of coupling the main rectangular waveguide to the four rectangular sections of the mode launcher;

FIG. 4 shows the construction of the power splitter; and

FIG. 5 is a crossover detail of the intercoupling waveguides in FIG. 3.

Refering to FIGS. 1, 2 and 3, the mode launcher to be described serves to directionally couple energy in main rectangular waveguide operating in the TE mode to the circular waveguide 11 operating in the TE mode. By directionally it is meant that no energy flows in that part of the circular waveguide to the left of the coupling as seen in FIG. 1. The coupling comprises four similar rectangular waveguide sections 12, 13, 14 and 15 spaced 90 apart around the circular waveguide 11 and passing through the wall of the circular waveguide. The longitudinal axes of the rectangular sections make an angle of 45 with the circular waveguide axis and intersect at the same point on the circular waveguide axis. The smaller dimension b of the rectangular sections is in a direction normal to the circular waveguide axis. The radius R of the circular guide and the larger dimension a of the rectangular sections are so related that the circular and rectangular guides have the same cut-off frequency. The dimension [2 is so selected that the characteristic impedance of the four rectangular sections in parallel equals the characteristic impedance of the circular waveguide.

The rectangular sections 12-15 extend inside the circular waveguide until the inner walls of b dimension intersect; however, the pyramid that would normally be formed by the intersection of these walls is omitted to provide an opening 16 for the passage of an electron beam 1-7 produced by electron gun 18, both shown dotted in FIG. 1. This construction is employed in certain traveling Wave tubes such as the Ubitron. A description of this tube may be found in an article by R. M. Phillips entitled, The Ubitron, a High-Power Traveling-Wave Tube Based on a Periodic Beam Interaction in Unloaded Waveguide, appearing in IRE Transactions on Electron Devices, volume ED-7, No. 4, October 1960. The principal difference between a conventional traveling wave tube and one of the Ubitron type is in the relative velocities of the interacting electromagnetic wave and electron beam. In the conventional tube, the wave is propagated through a loaded waveguide, usually referred to as the slow wave structure, designed to reduce the phase velocity of the electromagnetic wave to comparability with the electron velocity of the beam. In the Ubitron type, sometimes referred to as a fast wave tube, the waveguide in which the electromagnetic wave and the electron beam interact is unloaded so that the wave propagates at approximately the velocity of light while the electrons of the beam necessarily travel at a much lower velocity. In FIG. 1, 11 is the unloaded waveguide of the Ubitron into which the electromagnetic wave to be augmented is introduced by the mode launcher described herein. Under the proper conditions, described in the :above article, the Wave and the electron beam interact in waveguide 11 in such manner that some of the kinetic energy of the electrons in the beam is transferred to the wave thus increasing its energy level.

P or convenience in feeding, the rectangular sections 12- 15 external to the circular guide may be given H-plane bends to place their axes in a plane normal to the circular guide axis. In order to excite the TE mode in the circular waveguide the electric vectors E in the rectangular sections should be equal in magnitude and have the phase relation shown in FIG. 2. The apparatus for feeding the rectangular sections with energy of the proper amplitude and phase to accomplish this is shown in FIGS. 3, 4 and 5. The power in the main rectangular waveguide '10 is divided into four equal components by power splitter 20. The power splitter is made from two identical solid blocks of metal, one of which is shown in FIG. 4, in which the rectangular waveguide is machined to a depth of onehalf the greater dimension. The two halves are then clamped face-to-face. The existence of an imperfect contact between the two halves makes no difference since no current flows across this plane. There are three power halving forks in the power splitter with equal distances from the input 21, to which main waveguide 10 is coupled, to each of the four output ports 22, 23, 24 and 25. Consequently, one-fourth of the input power appears at each of the four ports and the waves at the four ports have undergone the same delays, so that the phase relations between the electric vectors at the ports are as shown by the arrows in FIG. 4. Therefore, if ports 22, 23, 24 and 25 are coupled to rectangular sections 12, 1'3, 14 and 15, respectively, by waveguides of equal length, the proper phasing, as shown in FIG. 2, for exciting the TE mode in the circular guide is obtained. Coupling waveguides 26, 27, 28 and 29, meeting this requirement, are shown in FIG. 3. FIG. 5 shows one type of crossover which may be used to maintain equality in the lengths of waveguides 26-29.

I claim:

1. Apparatus for directionally coupling high frequency energy from a rectangular waveguide operating in the TE mode to a circular waveguide operating in the TE mode, said apparatus comprising: four rectangular auxiliary waveguides of equal lengths each having at one end a straight portion which passes through the wall of said circular waveguide, said straight portions being equally spaced about said circular waveguide with their axes making an angle of'45 with the circular waveguide axis and intersecting the circular Waveguide axis at a common point, said straight portions extending inside said circular waveguide to the point of intersection with each other but not to the circular waveguide axis, the larger dimensions of said auxiliary waveguides being equal and chosen so that the cut-off frequencies of the auxiliary and circular waveguides are equal, and the smaller dimensions of said auxiliary waveguides being equal and chosen so that the characteristic impedance of the four auxiliary waveguides in parallel equals the characteristic impedance of the circular waveguide; and a power splitter having an input to which said rectangular waveguide is coupled and four out- .puts, said power splitter operating to divide the input energy equally between the four outputs and to provide equal delays between the input and each of the four outputs, the other ends of said auxiliary waveguides being coupled to the outputs of said power splitter in such order that the electric vectors at the ends of the auxiliary waveguides in said circular waveguide point in the same direction around a circle.

2. Apparatus for directionally coupling high frequency energy from a rectangular waveguide operating in the TE mode to a circular waveguide operating in the TE mode, said apparatus comprising: four rectangular auxiliary waveguides of equal lengths each having at one end a straight portion which passes through the Wall of said circular waveguide, said straight portions being equally spaced'about said circular waveguide with their axes making an angle of with the circular waveguide axis and intersecting the circular waveguide axis at a common point, said straight portions extending inside said circular waveguide to the point of intersection with each other but not to the circular Waveguide axis, the larger dimensions of said auxiliary waveguides being equal and chosen so that the cut-off frequencies of the auxiliary and circular waveguides are equal, and the smaller dimensions of said auxiliary waveguides being equal and chosen so that the characteristic impedance of the four auxiliary waveguides in parallel equals the characteristic impedance of the circular waveguide; and a coupling means connected between said rectangular waveguide and the other ends of said auxiliary waveguides, said coupling means dividing the power in said rectangular waveguide equally between said auxiliary waveguides and with such relative phases that the electric vectors at the auxiliary waveguide ends in said circular waveguide point in the same direction around a circle.

References Cited UNITED STATES PATENTS 2,686,901 8/1954 Dicke 333-21 2,800,632 7/1957 Walker 333- 21 3,230,484 1/1966 Lipetz et al. 333-21 HERMAN KARL SAALBACH, Primary Examiner. R. D. COHN, M. N-USSBA'UM, Assistant Examiners. 

1. APPRATUS FOR DIRECTIONALLY COUPLING HIGH FREQUENCY ENERGY FROM A RECTANGULAR WAVEGUIDE OPERATING IN THE TE10 MODE TO A CIRCULAR WAVEGUIDE OPERATING IN THE TE01 MODE, SAID APPARATUS COMPRISING: FOUR RECTANGULAR AUXILIARY WAVEGUIDES OF EQUAL LENGTHS EACH HAVING AT ONE END A STRAIGHT PORTION WHICH PASSES THROUGH THE WALL OF SAID CIRCULAR WAVEGUIDE, SAID STRAIGHT PORTIONS BEING EQUALLY SPACED ABOUT SAID CIRCULAR WAVEGUIDE WITH THEIR AXES MAKING AN ANGLE OF 45* WITH THE CIRCULAR WAVEGUIDE AXIS AND INTERSECTING THE CIRCULAR WAVEGUIDE AXIS AT A COMMON POINT, SAID STRAIGHT PORTIONS EXTENDING INSIDE SAID CIRCULAR WAVEGUIDE TO THE POINT OF INTERSECTION WITH EACH OTHER BUT NOT TO THE CIRCULAR WAVEGUIDE AXIS, THE LARGER DIMENSIONS OF SAID AUXIALIARY WAVEGUIDES BEING EQUAL AND CHOSEN SO THAT THE CUT-OFF FREQUENCIES OF THE AUXIALLY AND CIRCULAR WAVEGUIDES ARE EQUAL, AND THE SMALLER DIMENSIONS OF SAID AUXIALLY WAVEGUIDES BEING EQUAL AND CHOSEN SO THAT THE CHARACTERISTIC IMPEDANCE OF THE FOUR AUXIALLY WAVEGUIDES IN PARALLEL EQUALS THE CHARACTERISTIC IMPEDANCE OF THE CIRCULAR WAVEGUIDE; AND A POWER SPLITTER HAVING AN INPUT TO WHICH SAID RECTANGULAR WAVEGUIDE IS COUPLED AND FOUR OUTPUTS, SAID POWER SPLITTER OPERATING TO DIVIDE THE INPUT ENERGY EQUALLY BETWEEN THE FOUR OUTPUTS AND TO PROVIDE EQUAL DELAYS BETWEEN THE INPUT AND EACH OF THE FOUR FOUR OUTPUTS, THE OTHER ENDS OF SAID AUXIALLY WAVEGUIDES BEING COUPLED TO THE OUTPUTS OF SAID POWER SPLITTER IN SUCH ORDER THAT THE ELECTRIC VECTORS AT THE ENDS OF THE AUXILIARY WAVEGUIDES IN SAID CIRCULAR WAVEGUIDE POINT IN THE SAME DIRECTION AROUND A CIRCLE. 